The present invention relates to a ceramic heater: which can be mainly used in the semiconductor industry, the optical communication equipment industry, the electronic communication equipment industry and the like; and which is mainly used for heating a semiconductor wafer, or as an element for temperature adjustment for an optical device.
A semiconductor product is produced through the step of forming a photosensitive resin as an etching resist on a semiconductor wafer and subjecting the semiconductor wafer to etching, and the like steps.
This photosensitive resin is liquid, and is applied onto a surface of the semiconductor wafer, using a spin coater or the like. In order to scatter solvent and so on after the application, the resin must be dried. Thus, the semiconductor wafer subjected to the application is put on a heater and heated.
Hitherto, as a heater made of metal and used for such a purpose, a heater wherein heating elements are arranged on the back surface of an aluminum plate is adopted.
However, such a heater made of metal has the following problems.
First, the thickness of the heater must be as thick as about 15 mm since the heater is made of metal. This is because a warp, a strain and so on are generated in a thin metal plate because of thermal expansion resulting from heating so that a semiconductor wafer put on the metal plate is damaged or inclined. However, if the thickness of the heater is made thick, problems such that the heater becomes heavy and bulky arise.
Heating temperature is controlled by changing the voltage or amperage applied to the heating elements. However, if the metal plate is thick, the temperature of the heater plate does not follow the change in the voltage or amperage promptly. Thus, a problem that the temperature cannot be easily controlled is caused.
Thus, as described in Japanese Kokai Publication Hei 9-306642, Japanese Kokai Publication Hei 4-324276, Japanese Kokai Publication Hei 7-280462, U.S. Pat. No. 5,643,483 Specification and so on, ceramic heaters wherein AlN, which is a non-oxidized ceramic having a large thermal conductivity and a large strength, is used as a substrate and heating elements are formed on a surface of this AlN substrate or inside the AlN substrate are suggested.
Such a ceramic heater is usually used, being fitted into a supporting case through a seal ring.
However, in tests performed by the inventors, there arose an unexpected problem that uneven temperature portions were generated in the periphery of a wafer-heating surface of such a ceramic heater.
Also, particularly in the case that a ceramic substrate was placed without being fitted into the case, there arose a problem that the side face of the ceramic substrate was exposed so that small particles originated from ceramic grains fell down from the side face.
The problem of the unevenness of the temperature in the wafer-heating surface is considered to be related to a contact area between the side face and the seal ring. It is assumed that when the surface roughness of the side face is made too small, the contact area becomes large so that heat is conducted through the seal ring, thus a low-temperature area is generated in the periphery of the ceramic heater.
Thus, the inventors made eager investigations on the problems in the prior art. As a result, the inventors have found that when a ceramic substrate is fitted through a seal ring, the contact portion between the ceramic substrate and the seal ring can be made to be point-contact by making the side face of the ceramic substrate coarse to have a roughness within a given range, so that the temperature of the ceramic substrate can be made uniform.
The inventors have also found the following. If the surface roughness of the side face is too large, in the case that the side face of the ceramic substrate is exposed without contacting the seal ring, the side face exhibits an effect like a heat-radiating fin so that uneven temperature portions are generated in the periphery, also, small particles are generated. When the surface roughness is set within a given range, such a problem can be overcome. As a result, the present invention has been completed.
That is, the ceramic heater of the present invention is a ceramic heater, wherein a heating element is arranged on a surface of a ceramic substrate or inside the ceramic substrate, the surface roughness Rmax of the side face of the ceramic substrate being from 0.1 to 200 xcexcm according to JIS B 0601.
According to the ceramic heater of the present invention, its side face is made coarse to have a roughness within a given range. Therefore, when the ceramic substrate is fitted into a supporting case, the contact of the side face of the ceramic substrate with the supporting case is made to point-contact. Thus, conduction of heat from the ceramic heater to the supporting case is suppressed so that the temperature of the ceramic substrate can be made uniform.
If the surface roughness of the side face is too large, such an effect like a heat-radiating fin is exhibited even when the substrate contacts a seal ring. Thus, heat is radiated so that a low-temperature area is generated in the peripheral portion of the ceramic substrate. On the other hand, if the surface roughness is too small, the contact area with the seal ring becomes large. Also, even if the seal ring is not present, air which is contacting the side faces does not stay. Therefore, amount of heat-radiation is increased so that a low-temperature area is generated in the peripheral portion of the ceramic substrate.
In short, the inventors have found out that a specific range is present on the roughness of the side face of a ceramic substrate for reducing the amount of heat-radiation. In the present invention, if the temperature difference between the highest temperature and the lowest temperature of the heating surface of the ceramic substrate is about 0.5xc2x0 C. at 180xc2x0 C., about 8xc2x0 C. (2%) at 400xc2x0 C. and about 20xc2x0 C. (4%) at 500xc2x0 C., the ceramic substrate is considered to be practical. In order to achieve the above ranges of the temperature difference, the surface roughness Rmax of the side face of the ceramic substrate should be set to 0.1 to 200 xcexcm, and is desirably set to 0.5 to 200 xcexcm.
The above ceramic substrate has a high thermal conductivity, can cause the surface temperature of the heater plate to follow a change in the temperature of the heating element promptly, and can control the temperature in its wafer-heating surface satisfactorily. Additionally, the ceramic substrate has a great mechanical strength. Therefore, the heater plate is not warped so that a semiconductor wafer put thereon can be prevented from being damaged.
In the ceramic heater of the present invention, the ceramic substrate is desirably fitted into a supporting case, and is desirably made of a nitride ceramic, a carbide ceramic or an oxide ceramic.
The ceramic heater is desirably for heating a semiconductor wafer, and the thickness of the ceramic substrate is desirably over 1.5 mm.